The present invention relates in general to communication test systems and subsystems therefor, and is particularly directed to a frequency domain reflectometry-based test mechanism for measuring attenuation of a telecommunications wireline (e.g., a twisted metallic pair), and using the measured loss to estimate respective upstream and downstream bit rates at which digital subscriber line (DSL) type signals may be transported over the wireline.
The ability to conduct high data rate communications between remotely separated data processing systems and associated subsytems and components has become a requirement of a variety of industries and applications, such as business, educational, medical, financial and personal computer uses. Indeed, it can be expected that current and future applications of such communications will continue to engender more systems and services in this technology. Associated with such applications has been the growing use and popularity of the xe2x80x9cInternetxe2x80x9d, which continues to stimulate research and development of advanced data communications systems between remotely located computers, especially communications capable of achieving relatively high data rates over an existing signal transport infrastructure (e.g., legacy copper cable plant).
One technology that has gained particular interest in the telecommunication community is digital subscriber line (DSL) service, which enables a public service telephone network (PSTN) to deliver (over limited distances) relatively high bandwidth using conventional telephone company copper wireline infrastructure. DSL service has been categorized into several different technologies, based upon expected data transmission rate, the type and length of data transport medium, and schemes for encoding and decoding data.
Regardless of its application, the general architecture of a DSL system essentially corresponds to that diagrammatically shown in FIG. 1, wherein a pair of remotely separated, mutually compatible digital communication transceivers (e.g. modems) are coupled to a communication link 10, such as an existing copper plant. One of these transceivers, denoted as a xe2x80x98west sitexe2x80x99 DSL transceiver 1, is typically located at a network controller site 2 (such as a telephone company central office); the other, denoted as an xe2x80x98east sitexe2x80x99 DSL modem 3, may be coupled with a computer 9 located at a customer premises 4, such as a home or office.
Within the communication infrastructure of the telephone company, the xe2x80x98west sitexe2x80x99 DSL transceiver 1 is coupled with an associated xe2x80x98backbonexe2x80x99 network 5, which communicates with other data transport paths, by way of auxiliary equipments 6, such as routers and digital subscriber line access multiplexers (DSLAMs). Through these associated devices, the backbone network 5 may communicate with additional information sources 7 and the Internet 8. This telecommunication fabric thus allows information, such as Internet-sourced data (which is readily accessible via the backbone network 5), to be transmitted from the central office DSL transceiver 1 over the communication link 10 to the compatible DSL transceiver 3 at the customer site 4.
In a DSL system of the type described above, the data rates between DSL transceivers are considerably greater than those for voice modems. For example, while voice modems typically operate at a relatively low band, e.g., from near DC up to on the order of 4 kHz, DSL data transceivers may operate in a bandwidth between frequencies on the order of 125 kHz to well over 1 MHz. This voice/data bandwidth separation allows high rate data transmissions to be frequency division multiplexed with a separate voice channel over a common signal transport path.
Moreover, the high rate data DSL band may be asymmetrically, subdivided, as shown in FIG. 2, into an ADSL format, which allocates a larger (and higher frequency) portion of the available spectrum for xe2x80x98downstreamxe2x80x99 (west-to-east in FIG. 1) data transmissions from the central office site to the customer site, than data transmissions in the xe2x80x98upstreamxe2x80x99 direction (east-to-west in FIG. 1) from the customer site to the central office. As a non-limiting example, for the case of a single twisted copper pair, a bandwidth on the order of 25 kHz to 125 kHz may be used for upstream data transmissions, while a considerably wider bandwidth on the order of 130 kHz to 1.2 MHz may be used for downstream data transmissions. This asymmetrical downstream vs. upstream allocation of DSL bandwidth is based upon the fact that the amount of data transported from the central office to the customer (such as downloading relatively large blocks of data from the Internet) can expected to be considerably larger than the amount of information (typically email) that users will be uploading to the Internet.
Fortunately, this relatively wide separation between the upstream and downstream data bands facilitates filtering and cancellation of noise effects, such as echoes, by relatively simple bandpass filtering techniques. For example, an upstream echo of a downstream data transmission will be at the higher (downstream) frequency, when received at the central office, so as to enable the echo to be easily filtered from the lower (upstream) frequency signal. Asymmetric frequency division multiplexing also facilitates filtering of near-end crosstalk (NEXT), in much the same manner as echo cancellation.
In addition to performance considerations and limitations on the transport distance for DSL communications over a conventional twisted-pair infrastructure, the cost of the communication hardware is also a significant factor in the choice of what type of system to deploy. Indeed, a lower data rate DSL implementation may provide high-speed data communications, for example, at downstream data rates on the order of or exceeding 1 Mbps, over an existing twisted-pair and at a cost that is competitive with conventional non-DSL components, such as 56 k, V.34, and ISDN modems.
In an effort to optimize the bandwidth and digital signal transport distance of their very substantial existing copper plant (which was originally installed to carry nothing more than conventional analog (plain old telephone service or POTS) signals), telecommunication service providers may perform one or more test and measurement operations on the local loop (twisted wire pair), such as, but not limited to loop loss (attenuation), the presence of bridge-taps or load coils, and data integrity at various segments of cable plant.
Loop loss has been customarily measured by placing a signal transmitter at a first (near) end of the loop and a measurement device at a second (far) end of the loop. The signal generated by the transmitter, which may comprise a tone of known frequency and power, is received by the far end measurement device to determine the insertion loss across the bandwidth of interest for the service being deployed. The measured loop loss may then be compared with existing cable records or deployment guidelines for the network access equipment.
An obvious drawback to this measurement procedure is the need to employ two pieces of test equipment at opposite ends of the loop, which may be separated by miles of communication cable. Also, some test equipment is capable of generating only a limited set of tones, which may constrain testing capabilities for new services. For a given service, the network""s access equipment may assist in troubleshooting the local loop, as many different types of equipment are capable of estimating loop loss of signal power, which may be reported through a control port.
In accordance with the present invention, the need to interactively access and/or conduct a test message exchange session with a test unit installed at a far end of the loop is effectively obviated by a new and improved loop loss measurement mechanism, which employs single ended, frequency domain reflectometry (FDR)-based signal processing of the type disclosed in the above-referenced ""769 and ""681 applications. By differentially combining a distortion-corrected, normalized amplitude vs. frequency data array with an associated set of wireline noise spectrum values, and processing the resulting noise margin data by a Shannon Theorem operator, the FDR loop loss measurement technique obtains a set of N frequency bins, each containing the number of bits which the link will support for a respective tone. The bit contents of the bins which effectively represent a composite bit rate that is available for use for ADSL signalling.
For this purpose, the invention has a front end tone signal measurement section that includes a processor-controlled, swept tone-generating test head coupled to the wireline under test. As the test head outputs a sequence of digitally created discrete frequency sinusoidal tones, the wireline""s response signal level is monitored and digitized. In order to optimize the accuracy of the analysis, the response data is selectively filtered by a high Q, bandpass filter having a center frequency which is varied along the variation of frequency of the tone generator.
The filtered data is full wave rectified to derive root mean square (RMS) values of the signal amplitudes, which are stored with each frequency step iteration, thereby providing a sampled amplitude array of measurement points. The array of digitized amplitude samples produced by the front end, tone signal measurement section is normalized using stored reference data, values of which are derived by applying the tone signals to a known reference component, such as a prescribed value capacitor. This reference is used to simulate the line loss characteristics, which are dependent upon wire gauge number, type of dielectric (insulation), etc.
The normalized data samples are coupled to a distortion correction operator, which corrects for cable feed extensions, the most common form of which are bridged taps, that impact ADSL signal through constructive and destructive interference, manifested as peaks and valleys in the loop loss characteristic. Using the average gain slope from the normalized loss characteristic, peaks above the gain slope line/axis are folded about that axis, so as to produce a set of associated valleys below the axis. Data points more than a prescribed differential off-axis are modified by a correction factor, that is adjustable to accommodate for various cable characteristics, to produce corrected data points.
The distortion-corrected normalized data is differentially combined with an array of power spectral density values associated with the type of modem at the far end of the link. The resulting modem-modified differential data is summed with a signal-to-noise gap constant required for reliable operation and coding gain values associated with the coding gain of the modem transceivers. The coding gain-modified data set is combined in a differential combiner.
A noise spectrum measurement digitizer is coupled to an associated sense amplifier circuit coupled to the wireline, and provides digitized data representative of the line""s background noise. The digitized wideband noise floor spectrum values contain N noise values for successive frequency steps having associated tone response data values in the swept tone amplitude response array. These noise values are subtracted from the coding gain modified data set to produce an array of noise margin values. This noise margin data set is processed in a Shannon Theorem operator, the output of which is a set of N frequency bins, each containing the number of bits which the link will support for that tone.
The total number of bits for the N frequency bins effectively represent a composite bit rate that is available for use for upstream and downstream signalling among a variety of DSL formats. The bits/tone composite is coupled to a bin selection operator which selectively subdivides the bits/tone total into upstream and downstream totals in accordance with a user-selected DSL allocation. The upstream and downstream bit rate totals provide estimates of the maximum bit rates at which ADSL signaling may be conducted over the wireline.